DIY Video Projector

[Darth Willis]

Very true tech head, but I noticed that even those screens still suffer from the hot center and dim edges... that's why I don't think they ever caught on to begin with. Too low an image quality... this is yet another reason why I don't see it as worth it. Brute forcing your way around limitations might work to a degree, but it's no substitute for doing it right.

To make it worth doing, the end result in my opinion needs to have a contrast ratio around 200:1 minimum and an image resolution of at least 800x600 native... but YMMV. Again, I'm one of those looking at wanting professional results.

[SuperDave]

I did think about using the chrome paint...However, I was told and I also read somewhere that the chrome paint will sometimes look OK if you do and EXCELLENT job spraying...but if you try to polish it or even clearcoat it, it turns into a very dull grey. I have used a cheap can about 8 years ago when I was in highschool and I though I was going to paint my bike chrome (HA what a disaster). I think I might go out and get a decent can and see what I can do if it works that will be Great for what I want to do.

Now remember...for the time being I dont care if I have a ugly beast of a projector...I can make it pretty and small later...What I want to do is use the technique for drawing an elipse around 2 focal points. I will sketch this onto a thin piece of plywood and cut out a flat, eliptical shape. If I can get a bucket of bondo or other material I can set it up so I can rotate the plywood elipse circularly inside the bondo mixture and after a while have a fairly accurate ellipsoidal reflctor cast. Then, depending on if I make a positive or negative mold, I can either reuse it to mold plastic reflectors on it or just paint it with something to make it reflective and use it.

I would like to create a negative mold of the ellipse so I can then use it to pour plastic (or other material...I would still like info or suggestions) over it to form new reflectors. So.. if anyone has info on what substance I can use...or can provide some useful links I would be grateful.

Thanks,
Dave

[tech head]

SuperDave

check your e-mail.

[Axeman]

http://www.projectorpeople.com/news_info/ppeople.asp

[jjasniew]

The OHP with 7 K lumens I have is the ELMO HP-A305. Virtually the same thing is for sale now on ebay; see my earlier post.

I think the image was about 60" on a diag. That dimension seemed to be the best balance between brightness and size at the time.

This image was in a dead-dark room, with even the ambient light spilling out of the projector vents competing with the projection.

There's not much hope of getting a trully impressive bright image with the room lights on. Certainly, it can be made better with a high gain screen (expensive) or a high gain rear projection screen (expensive). By expensive I mean a few times the cost of the entire thing just for the screen.

To each his or her own; I wasnt satisfied and went ahead and bought a CRT unit for 1K$. Another person I know sets up an entire room in their home, dedicated to a 640X480 1.4M color panel with a 4K lumen OHP, that they paid me $200 for -

[Darth Willis]

Hmmm, didn't think about that Dave... never bothered to try, and had heard of limited success. Oh well... perhaps if you tried painting the opposite side of the plastic instead or maybe tried a high gloss finish white. *lobbing out ideas*

[jjasniew]

I dont see how the OHP lens system is much different than a camera.

Say you put a bellows from the lens on down to the stage, so that the only light at the stage has to come through the lens. Then, a flat photographic film plate where the transparency goes...

Presume it's focused on some large drawing on a wall and there's a lens cap so as not to expose the film. In a pitch dark room, pop off the cap and touch off a flash to make a "bulb" exposure, as you might do by holding the shutter open with a film camera.

Whatever drawing this thing was focused on, should now be exposed on the film-plate. Correct?

[tech head]

You are absolutly right jjasniew. But the problem isn't focusing the image. the problem is getting enough of the right kind of light through the lcd all going in the right direction.

[tech head]

Another question for Darth.
You said that brute force is not the way. Isn't that what the big pj companies do with their 10000 lumen lights and itty bitty lcd's?

[Darth Willis]

Not from what I've seen, tech head. What the actual projectors are using from what I understand is three seperate mono LCDs for RGB where the source light is being split up, filtered and reassembled. It's a lot easier to work with LCD panels that small because the optics needed at that size are cheaper and easier to work with, plus the LCDs are thinner and specifically designed for this purpose. I could be wrong, but I don't think the actual bulbs in those projectors are much brighter than about 4500 lumens or so for the ultra brights. They can't be too bright or powerful due to obvious heat and size issues. If you can prove me wrong, please do... I don't have one to tear apart, but it seems like the cheapest, easiest route to take and keep a reasonable profit margin. LCD manufacturing can always improve and get cheaper, large size glass lens making will always be expensive. Improve what you can and accept the limitations of the other parts on a size/scale/cost ratio.

I'm simply trying to approach the problem from a logical standpoint. The problems at hand almost all center around light usage, efficiency and distortion. Lumens calculations are simple elementary school math. As long as all of your optics are designed to utilize as much light as possible and keep it focused from one point to the next, you should be rocking. I'm not sure what kind of light loss is on a large LCD panel, but I have a hard time believing it's 90%... I have a feeling it's closer to around 20% if you do things right. If you figure a generous 5-10% loss per lens, only two to three lenses... you should be able to get at least 60% of your light out the end. You have an even 4000 lumens light source properly focused and sized, you should be getting around 2000+ from the projection. That's a daylight usable brightness. As for distortion, removal of that will only come through higher quality lenses.

I'd do this project and be happy with only about a 95% correct geometry... it'd irritate me a bit, but can be forgiven more than a drastically weak, fuzzy, unevenly lit image. Plus, considering the monetary value needed to invest in the proper lenses, once you cured the lighting situation, most of your geometry would improve as well.

The only problem with using OHPs is they're designed to be sloppy and low tech with a lot of wiggle room. You can do it with that kind of setup, but it'd probably be best to replace all the lenses with better quality ones.

jjasniew, the bellows are there to keep ambient light from washing out the image. It would be an excellent idea to encase the entire stretch of projector and light seal all the crucial bits until it came out the end to get the best contrast depth possible, but is again something that shouldn't be worried about until the optics are figured out and optimised.

Something to remember, most all the parts that are being used aren't designed with these purposes in mind. In order to use them to their fullest potential, you have to improve the quality of the parts you have control over to compensate for the ones you don't have control over. The limitations are already known, light works the same no matter where you are in this universe, think about what you're doing and plan a little before diving further in to tweak. Treat the optics like a street system in a downtown city. Yes three lefts do equal a right, but you wind up driving four extra blocks just to do it. You know where you're going, you just need to look at the map to find the shortest route. After all, the shortest route is almost always the cheapest, easiest and most successful.

&lt/cheezy fortune cookie advice&gt